Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO$_{3}$/SrTiO$_{3}$ superlattices

TL;DR

This study uses first-principles calculations to explore how strain, electrostatic conditions, and octahedral rotations interact in PbTiO₃/SrTiO₃ superlattices, revealing control mechanisms for polarization orientation and enhanced piezoelectric responses.

Contribution

It uncovers the coupling between ferroelectric and antiferrodistortive modes and proposes a covalent model to explain this interaction, which varies with superlattice periodicity.

Findings

Large piezoelectric responses at low epitaxial strains.

Strong coupling between FE and AFD modes.

Covalent interactions explain FE-AFD coupling beyond steric arguments.

Abstract

We report first-principles calculations on the coupling between epitaxial strain, polarization, and oxygen octahedra rotations in monodomain (PbTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_{n}$ superlattices. We show how the interplay between (i) the epitaxial strain and (ii) the electrostatic conditions, can be used to control the orientation of the main axis of the system. The electrostatic constrains at the interface facilitate the rotation of the polarization and, as a consequence, we predict large piezoelectric responses at epitaxial strains smaller than those that would be required considering only strain effects. In addition, ferroelectric (FE) and antiferrodistortive (AFD) modes are strongly coupled. Usual steric arguments cannot explain this coupling and a covalent model is proposed to account for it. The energy gain due to the FE-AFD coupling decreases with the periodicity of the superlattice, becoming negligible for $n \ge 3$.